its too late it takes 30 years to make a 30 year old, the decline is terminal
Eh, not really. Like, looking at the 2020 China tree, the Chinese don't really reach the end of their population boom for about 60 years (their largest population generation is still in their early 30s).
And I mean, if all they managed to do was stabilize the generations at about 40 million per age bracket above, in 30 years the 30-60 pop is still going to be half a billion working age adults: if the current 30-60s population is undiminished when its the 60-90 pop, that would be about 540 million. Realistically, post 60 you generally start getting quite a bit of die off, so really your talking about about 1/3 that, or about 200 million. This by modern figuring would be a support burden of about 2.4 to 1, but, well, that's also pretty normal and in line with, well, a stable population.
If China stabilized on a population of 500 million 30-60 year olds, you would expect there to be about 500 million 0-30 year olds, and probably 200 million 60-90 year olds. And, well, a china that "stabilized" around 1.2 billion, 200 million less than they currently have, would probably be a happier country.
The danger of a slightly shrinking Chinese population over the course of 40 years is I think a bit overstated. All they really need for a potentially happy country is aproximately maintaining grades about as big as the current ones.